Light source intensity control systems and methods for improved light scattering polarimetry measurements

Abstract

Systems and methods of performing a stress measurement of a chemically strengthened glass using a light-scattering polarimetry system include adjusting the intensity of a light beam from a light source in an illumination system using a rotatable half-wave plate and a first polarizer operably disposed between the light source and a rotating light diffuser that has a rotation time t R . The first polarizer is aligned with a second polarizer in a downstream optical compensator to have matching polarization directions by rotating the rotatable half-wave plate to a position where the exposure time t E falls within an exposure range t R ≤t E . The method also includes performing an exposure using the exposure time t E to obtain the stress measurement. One or both of the half-wave plate and first polarizer can be tilted to avoid deleterious back-reflected light from entering the light source.

What is claimed is: 
     
       1. A method of performing a stress measurement of a chemically strengthened glass using a light-scattering polarimetry (LSP) system having a light source system that emits a light beam, an optical compensator and a digital detector with an integration time t I , comprising:
 a) adjusting the intensity of a light beam using a rotatable half-wave plate and a first polarizer operably disposed between the light source and a rotating light diffuser that has a rotation time t R  by aligning the first polarizer with a second polarizer in the optical compensator to have matching polarization directions by rotating the rotatable half-wave plate to a position where the exposure time t E  falls within an exposure range t R ≤t E ; and 
 b) performing an exposure using the exposure time t E  to obtain the stress measurement. 
 
     
     
       2. The method according to  claim 1 , wherein the chemically strengthened glass comprises a first chemically strengthened substrate having a first amount of light scattering and further comprising:
 replacing the first chemically strengthened substrate in the LSP system with a second chemically strengthened substrate having a second amount of light scattering that is different from the first amount of light scattering by at least a factor of 2×; and 
 repeating acts a) and b) for the second chemically strengthened substrate. 
 
     
     
       3. The method according to  claim 1 , wherein said adjusting comprises measuring a contrast of an intensity distribution of an LSP image captured by the digital detector. 
     
     
       4. The method according to  claim 1 , wherein the exposure time t E  is between 5 ms and 10 ms. 
     
     
       5. The method according to  claim 1 , wherein the light source system comprises an axis along which the light beam travels, wherein the rotatable half-wave plate and the polarizer form back-reflected light from the light beam, and further comprising operably tilting at least one of the rotatable half-wave plate and first polarizer relative to the axis to avoid directing the back-reflected light to the light source. 
     
     
       6. The method according to  claim 1 , wherein the rotatable half-wave plate has a tilt angle θ WP  in the range 10°≤θ WP ≤20° relative to the axis. 
     
     
       7. The method according to  claim 1 , wherein the polarizer has a tilt angle θ P  in the range 5°≤θ P ≤10° relative to the axis. 
     
     
       8. The method according to  claim 1 , further comprising rotating the rotatable half-wave plate with a motorized mount that operably supports the rotatable half-wave plate. 
     
     
       9. The method according to  claim 8 , wherein the rotating the rotatable half-wave plate is conducted with a controller configured to automatically operate the motorized mount. 
     
     
       10. The method according to  claim 1 , further comprising:
 forming a digital LSP image of the chemically strengthened glass on a digital detector having a saturation limit and comprising pixels having corresponding pixel intensities as defined by the digital LSP image; and 
 wherein said adjusting of the intensity of the light beam comprises limiting the pixel intensities to be between 25% and 75% of the saturation limit. 
 
     
     
       11. An illumination system for providing light intensity control in a light scattering polarimetry (LSP) system for measuring stress in a chemically strengthened glass and in optical communication with an optical compensator and a digital detector with an integration time t I  and comprising in order along an axis:
 a light source that emits a light beam along the axis at a first intensity; 
 a rotatable half-wave plate; 
 a first polarizer having first polarization direction aligned with a second polarization direction of a second polarizer in the optical compensator; 
 a rotatable light diffuser that has a rotation time of t R ; and 
 wherein the rotatable half-wave plate is set at a rotation angle such that the light beam exiting the first polarizer and incident upon the rotatable light diffuser has a second intensity less than the first intensity that causes the LSP system to have an exposure time t E  for the CS glass that falls within an exposure range t R ≤t E  for measuring the stress in the CS glass. 
 
     
     
       12. The illumination system according to  claim 11 , wherein the rotatable half-wave plate and the polarizer form back-reflected light and wherein at least one of the rotatable half-wave plate and the polarizer is/are operably tilted relative to the axis to direct the back-reflected light to the light source. 
     
     
       13. The illumination system according to  claim 11 , wherein the rotatable half-wave plate has a tilt angle θ WP  in the range 10°≤θ WP ≤20° relative to the axis. 
     
     
       14. The illumination system according to  claim 11 , wherein the polarizer has a tilt angle θ P  in the range 5°≤θ P ≤10° relative to the axis. 
     
     
       15. The illumination system according to  claim 11 , wherein the exposure time t E  is between 5 ms and 10 ms. 
     
     
       16. The illumination system according to  claim 11 , wherein the light source comprises a laser diode having an optical power in the range from between 20 and 300 milliwatts. 
     
     
       17. The illumination system according to  claim 11 , wherein the light diffuser comprises a holographic light diffuser. 
     
     
       18. The illumination system according to  claim 11 , wherein the second polarizer comprises a polarizing beam splitter. 
     
     
       19. The illumination system according to  claim 11 , further comprising a rotatable motor mount that rotatably supports the rotatable half-wave plate. 
     
     
       20. The illumination system according to  claim 19 , further comprising a controller configured to automatically operate the rotatable motor mount.